Discussion
High variation of sizes, shapes, and p/ D ratios were observed in
California Quercus species. The differences of sizes between
evergreen and deciduous oak pollen were found in California species. The
shorter furrows in deciduous oak pollen than that in evergreen oak
pollen was observed. Quercus douglasii , Quercus garryana ,
and Quercus kelloggii display higher p/D ratios than other
species. These 3 species are coincided with 3/4 deciduous oaks (not
including Quercus Xmacdonaldii ) in samples. The relationship
between this feature and deciduous leaves or dry environment adaptation
is not clear.
The variation of sizes, shapes, and surface ornamentation of pollen inQuercus section is the highest and average pollen size is the
largest in three sections. That might be resulted from the larger number
of species categorized in this section. New World Quercussection, Protobalanus , has the smallest average size and
variation in three studied sections. This group is unique by its smooth
pollen surface, dense scabrate/rodlike masked (Denk and Grimm, 2009).
The higher similarity of sizes between Quercus and Lobataesections coincide with the closer phylogenetic distance of these two
sections than them and Protobalanus (Figure 1 and 2; Mano et al.,
2001; Denk and Grimm, 2009). We applied T-test on 20 species (excluding
hybrid Quercus , Chrysolepis , and Notholithocarpus ).
The results suggest that there are significant different between sizes
and shapes of each species, each section, and evergreen-decidous oaks. I applied Moran’s tests on 20 species to test phylogenetic signals.
There is only the length of polar view passed Moran’s test
(p <0.05; p = 0.041 and 0.042 in 2 tests) when I tested 20 species at the same time. It suggested that the length of
polar view of the oak pollen grains contain phylogenetic signals. I also found phylogenetic signal in the distance between 2 furrows in
Lobatae sections, Q. agrifolia, Q. kelloggii, Q. parvula, and Q.
wislizeni (p <0.05; p =0.035). Further
investigations will be focused on details of surface ultrastructure and
trait-environment relationships.
Scanning electron microscopy (SEM) images provide details of surface
patterns of Quercus pollen. Former SEM studies about oak pollen
in the United States only display comparison of Fagaceae phylogenies and
several images of pollen structure, such as Solomon’s (1983) pollen
morphologic studies about white and red oaks in eastern North America
and Médus’ and Flores’ (1984) pollen morphology of some Mexican oaks.
Based on the terminology and classification from Denk’s and Grimm’s
(2009), oak pollen grains in Quercus and Lobatae sections
in this study can be classified into “(micro) verrucate” type. Quercus cedrosensis , Quercus chrysolepis , Quercus
palmeri , Quercus tomentella , Quercus parvula , and Quercus wislizeni from Protobalanus were classified into
“rodlike masked” type (Figure 10).
Because of the gradually changes in rodlike masked sculpture, Quercus pollen surface ornamentation of studied species were be
able to classified into another 3 types. Type 1 includes Q.
agrifolia, Q. cornelius-mulleri, Q. dumosa, Q. durata, Q. engelmannii,
and Q. Xmacdonaldii. Type 2 includes Q. kelloggii, Q. turbinella, Q.
pacifica, Q. garryana, Q. john-tuckeri, Q. lobate, Q. wislizeni, Q.
palmeri, Q. parvula, Q. saddleriana, Q. douglasii, and Q. Xalvordiana.
Type 3 includes Q. cedrosensis, Q. vaccinifolia, Q. chrysolepis, and Q.
tomentella. The surface elements on Type 1 Quercus pollen grains
are blocks (verrucate) with sparse micro-elements on them. Type 2 pollen
grains posse big blocks with dense micro-elements on them. Type 1 and
type 2 are both included to “(micro) verrucate” category in Denk’s and
Grimm’s classification. Type 3 can be matched to rodlike masked category
(Figure 10).
The pollen morphologies of Chrysolepis chrysophylla and Notholithocarpus densiflorus var echinoides ,
support their closer phylogenetic relationships than them to Quercus genus. High endemism of their geographical distribution
provides evidence for the relictual nature of the castaneoids occurring
in western North America. The differential pollen morphologies coincide
with their insect pollination mechanism which is different to wind
pollination of Quercus .